The present invention relates to a color image forming apparatus employing an electro-photographic method, such as a copier, a printer, a facsimile and a complex image forming apparatus having the abovementioned functions, and specifically relates to a color image forming apparatus provided with an intermediate transfer member on which a plurality of unicolor toner images are overlapped with each other to form a full color image.
As the color image forming apparatus employing the intermediate transfer member, there has been well-known a color image forming apparatus in which toner images formed on image bearing members, each serving as a photosensitive material, are transferred onto the intermediate transfer member, and then, the toner image residing on the intermediate transfer member is further transferred onto a transfer material (hereinafter, also referred to as a paper, a sheet, or a sheet paper). In such the color image forming apparatus, the toner images, which are sequentially formed on the image bearing members and electronically charged in a predetermined polarity, are electro-statically transferred onto the intermediate transfer member by overlapping them with each other, and then, the full color toner image formed on the intermediate transfer member is further electro-statically transferred onto the transfer material at a time.
Since, in the image forming apparatus employing the intermediate transfer member, the toner images formed on the image bearing member can be overlapped onto the intermediate transfer member, such the image forming method is widely employed for the color image forming apparatus, which forms a color image on the transfer material. In the color image forming apparatus mentioned in the above, the different unicolor toner images respectively formed on the image bearing members are transferred onto the intermediate transfer member by overlapping them with each other, and then, the overlapped toner image is electrostatically transferred onto the transfer material at a time.
Since a charge amount per one particle of toner is substantially uniform, an electric potential of a toner layer on the intermediate transfer member is determined by an amount of toner attached. Accordingly, in the color image forming apparatus, the electric charge potential of an area where a plurality of unicolor toner images are overlapped with each other is higher than that of another area where a single unicolor toner image is attached, within the toner image formed on the intermediate transfer member. Further, for instance, when the toner image formed on the intermediate transfer member includes both a solid tone area and a halftone area, the electric charge potential of the solid tone area is greater than that of the halftone area.
Further, variations of the electric charge potential within the toner image, which has passed through a primary transferring section for transferring the toner image to the intermediate transfer member from the image bearing member are possibly generated depending on the environmental factor.
As mentioned in the above, a large variety of the electric charge potential within the toner image residing on the intermediate transfer member causes variations of transfer characteristics corresponding to various areas within the same toner image area. When partial toner images corresponding to such the various areas having transfer characteristics different from each other are tried to be transferred onto the transfer material under the same transferring condition, various kinds of image defects are liable to occur when performing the secondary transferring operation from the intermediate transfer member to the transfer material.
In recent years, the coloring trend has been progressed in the field of a copier, a printer, a facsimile, a complex image forming apparatus having such the functions, etc., and the demand for high image quality is getting large in the field of transferring process by employing polymerized toner or small-sized particle toner. Further, the speed-up trend of the image forming apparatus has been also progressed. Considering such the trends, to obtain a good image, it is necessary to compensate for the toner potential residing on the intermediate transfer member at a substantially uniform potential, which is liable to change depending on a number of primary transferring operations and the environmental factors, so as to improve an efficiency of the secondary transferring operation.
To overcome the abovementioned problems, Patent Document 1 sets forth a configuration in which a charging device before transfer for charging the toner image, which is primary-transferred onto the intermediate transfer member and to be transferred onto the transfer material, is provided, and a conductive roller member is disposed at rear side of the intermediate transfer member so that the electrode of the charging device before transfer is disposed opposite to the conductive roller member while putting the intermediate transfer member between them. According to the abovementioned method, the charging amount of the toner image transferred on the intermediate transfer member is made to be substantially uniform by applying AC and/or DC corona discharging operation to the toner image.
Patent Document 2 sets forth a method of equipping a controlling device for controlling the charging condition for the charging device before transfer, corresponding to the moving velocity of the intermediate transfer member, which passes through a charging point of the charging device before transfer.
Patent Document 3 sets forth a secondary transferring device, which includes a first member and a second member, both of which are made of conductive urethan rubber foam and volume resistivity of which are different from each other. At a time of room temperature, the first member press-contacts the intermediate transfer member, while, at a time of high temperature and high humidity, the second member press-contacts the intermediate transfer member.                [Patent Document 1]                    Tokkaihei 10-274892 (Japanese Non-Examined Patent Publication)                        [Patent Document 2]                    Tokkaihei 11-143255 (Japanese Non-Examined Patent Publication)                        [Patent Document 3]                    Tokkaihei 11-52763 (Japanese Non-Examined Patent Publication)                        
To cope with the demand for the speed-up trend of the printing velocity, it is necessary to increase the contact area between the transferring roller and the intermediate transfer member, in order not to decrease the transferring efficiency. For this purpose, various countermeasures, such as an increase of the press-pressure of the transferring roller, a lowered elasticity of the sponge of the transferring roller, an increase of the diameter of the transferring roller, etc., have been conventionally employed.
However, when the countermeasure(s) of increasing the press-pressure of the transferring roller and/or lowering elasticity of the sponge of the transferring roller are/is employed, an increase of replacing frequency of the transferring roller due to the low durability of the transferring roller would arise as another problem. Further, when the countermeasure of increasing the diameter of the transferring roller is employed, an increase of apparatus size and an increase of manufacturing cost would arise as another problem.
In the method set forth in Patent Document 1 and Patent Document 2, since the charge amount of toner on the intermediate transfer member is uniformed at a relatively large value, image defects, caused by discharging action due to a potential elevation of the paper when the resistivity of the paper is high due to a low humidity environment or at the time of transferring the second surface of the paper in the duplex copy mode, are liable to occur. If the transferring voltage is kept at low value in order to prevent such the image defects, the transferring electric field would be short at a partial area where the total charge amount of the toner layer is large, resulting in a density unevenness in a reproduced image.
Further, in the configuration, set forth in Patent Document 1 and Patent Document 2, in which the conductive roller member is disposed at rear side of the intermediate transfer member so that the electrode of the charging device before transfer is disposed opposite to the conductive roller member while putting the intermediate transfer member between them, since the line velocity of the intermediate transfer member becomes high in the high-speed image forming apparatus, it becomes difficult to acquire a sufficient charging effect, and further, the configuration for controlling the charging condition, etc. corresponding to the line velocity of the intermediate transfer member would become complex.
In the method set forth in Patent Document 1, although the transferring operation is achieved by directly contacting the secondary transferring device, which includes the first member and the second member, both of which are made of the conductive urethan rubber foam, there has been a problem that it is difficult to maintain initial efficiencies of the image forming apparatus, since paper particles caused by contacting actions or abrasion moving actions between the secondary transferring device and the transfer material adhere to the surface of the secondary transferring device.
To overcome the abovementioned drawbacks in conventional image forming apparatus, it has been desired to provide an image forming apparatus, in which the transferring efficiency is improved in order to acquire a high quality image and the discharging function before the secondary transfer, which makes the high durable transferring operation possible, is equipped.